Reduction of oxides of nitrogen in a gas stream using molecular sieve SSZ-28

ABSTRACT

The invention relates generally to molecular sieve SSZ-28 and its use in the reduction of oxides of nitrogen in a gas stream such as the exhaust from an internal combustion engine.

TECHNICAL FIELD

The invention relates generally to molecular sieve SSZ-28 and its use inthe reduction of oxides of nitrogen in a gas stream.

BACKGROUND

Because of their unique sieving characteristics, as well as theircatalytic properties, crystalline molecular sieves and zeolites areespecially useful in applications such as hydrocarbon conversion, gasdrying and separation. Although many different crystalline molecularsieves have been disclosed, there is a continuing need for new molecularsieves with desirable properties for gas separation and, drying,hydrocarbon and chemical conversions, and other applications.

SUMMARY

In accordance with this invention, there is provided a process for thereduction of oxides of nitrogen contained in a gas stream wherein theprocess comprises contacting the gas stream with a crystalline molecularsieve having a mole ratio of an oxide selected from silicon oxide,germanium oxide and mixtures thereof to an oxide selected from aluminumoxide, gallium oxide, iron oxide, boron oxide and mixtures thereofgreater than about 20:1 to about 45:1. The molecular sieve has, aftercalcination, the X-ray diffraction lines of Table 2. The molecular sievemay contain a metal or metal ions (e.g., cobalt, copper, platinum, iron,chromium, manganese, nickel, zinc, lanthanum, palladium, rhodium ormixtures thereof) capable of catalyzing the reduction of the oxides ofnitrogen, and the process may be conducted in the presence of astoichiometric excess of oxygen. In one embodiment, the gas stream isthe exhaust stream of an internal combustion engine.

DETAILED DESCRIPTION

The present invention comprises a molecular sieve designated herein“molecular sieve SSZ-28” or simply “SSZ-28.” Molecular sieve SSZ-28 andmethods for its preparation are disclosed in U.S. Pat. Nos. 5,200,377and 5,785,947.

In preparing SSZ-28, an N,N-dimethyltropinium or anN,N-dimethyl-3-azonium bicyclo[3.2.2]nonane cation can be used astructure directing agent (“SDA”), also known as a crystallizationtemplate. The SDAs useful for making SSZ-28 are represented by thefollowing structures (1) and (2):

The SDA cation is associated with an anion which can be any anion thatis not detrimental to the formation of the SSZ-28. Representative anionsinclude halogen (e.g., fluoride, chloride, bromide and iodide),hydroxide, acetate, sulfate, tetrafluoroborate, carboxylate, and thelike. The SDA can be used to provide hydroxide ions. Thus, it can bebeneficial to ion exchange, for example, a halide to hydroxide ion.

In general, SSZ-28 is prepared by contacting, in the presence ofhydroxide ion, (1) an oxide selected from silicon oxide, germanium oxideand mixtures thereof, (2) an oxide selected from aluminum oxide, galliumoxide, iron oxide, boron oxide and mixtures thereof, (3) a structuredirecting agent selected from N,N-dimethyltropinium andN,N-dimethyl-3-azonium bicyclo[3.2.2]nonane cations.

SSZ-28 is prepared from a reaction mixture comprising, in terms of moleratios, the following:

Typical Exemplary YO₂/X₂O₃   20 to <50   30 to 45 OH⁻/YO₂ 0.10 to 0.450.20 to 0.40 Q/YO₂ 0.05 to 0.50 0.10 to 0.20 M⁺/YO₂ 0.05 to 0.30 0.15 to0.30 H₂O/YO₂   20 to 300   25 to 60 Z/YO₂   0 to 1.0 0.20 to 0.40wherein Y is selected from silicon, germanium and mixtures thereof; X isselected from aluminum, gallium, iron, boron and mixtures thereof; Q isa is a structure directing agent selected from N,N-dimethyltropinium andN,N-dimethyl-3-azonium bicyclo[3.2.2]nonane cations; M is an alkalimetal cation; and Z is an amine component comprising at least one aminechosen from amines having from 1 to 8 carbon atoms, ammonium hydroxideand mixtures thereof.

Typical sources of aluminum oxide include aluminates, alumina, andaluminum compounds such as AlCl₃, Al₂(SO₄)₃, Al(OH)₃, kaolin clays, andother zeolites. An example of the source of aluminum oxide is LZ-210zeolite (a type of Y zeolite).

Typical sources of silicon oxide include silicates, silica hydrogel,silicic acid, colloidal silica, fumed silica, tetraalkyl orthosilicatesand silica hydroxides. Gallium, iron, boron and germanium can be addedin forms corresponding to their aluminum and silicon counterparts.Salts, particularly alkali metal halides such as sodium chloride, can beadded to or formed in the reaction mixture.

The reaction mixture can optionally comprise an amine component (Z)comprising at least one amine chosen from amines having from 1 to 8carbon atoms, ammonium hydroxide and mixtures thereof. Non-limitingexamples of these amines include isopropylamine, isobutylamine,n-butylamine, piperidine, 4-methylpiperidine, cyclohexylamine,tert-octylamine, cyclopentylamine and mixtures thereof. The use of theseamines can permit a reduction in the amount of the structure directingagent used resulting in a significant cost savings. By using the aminecomponent, the amount of the structure directing agent can be reduced toa level below that which is required to fill the micropore volume of themolecular sieve, i.e., an amount less than required to crystallize themolecular sieve in the absence of the amine component. In addition, theuse of the amine component can promote faster crystal growth when usedin combination with seed crystals. Methods for preparing SSZ-28 using anamine component are disclosed in U.S. Pat. No. 5,785,947.

In practice, SSZ-28 is prepared by a process comprising: (a) preparingan aqueous solution comprising (1) an oxide selected from silicon oxide,germanium oxide and mixtures thereof, (2) an oxide selected fromaluminum oxide, gallium oxide, iron oxide, boron oxide and mixturesthereof, (3) a structure directing agent selected fromN,N-dimethyltropinium and N,N-dimethyl-3-azonium bicyclo[3.2.2]nonanecations having an anionic counterion which is not detrimental to theformation of SSZ-28 and (4) an alkali metal cation; (b) maintaining theaqueous solution under conditions sufficient to form crystals of SSZ-28;and (c) recovering the crystals of SSZ-28.

The reaction mixture is maintained at an elevated temperature until thecrystals of the SSZ-28 are formed. The hydrothermal crystallization isusually conducted under autogenous pressure, at a temperature between100° C. and 200° C., typically between 135° C. and 180° C. Thecrystallization period is usually greater than 1 day and typically fromabout 5 days to about 10 days. The molecular sieve may be prepared usingmild stirring or agitation.

During the hydrothermal crystallization step, the SSZ-28 crystals can beallowed to nucleate spontaneously from the reaction mixture. The use ofSSZ-28 crystals as seed material can be advantageous in decreasing thetime necessary for complete crystallization to occur. In addition,seeding can lead to an increased purity of the product obtained bypromoting the nucleation and/or formation of SSZ-28 over any undesiredphases. When used as seeds, SSZ-28 crystals are added in an amountbetween 0.1 and 10% of the weight of the oxide selected from siliconoxide, germanium oxide and mixtures thereof that is used in the reactionmixture.

Once the molecular sieve crystals have formed, the solid product isseparated from the reaction mixture by standard mechanical separationtechniques such as filtration. The crystals are water-washed and thendried, e.g., at 90° C. to 150° C. for from 8 to 24 hours, to obtain theas-synthesized SSZ-28 crystals. The drying step can be performed atatmospheric pressure or under vacuum.

SSZ-28 has a composition, as-synthesized (i.e. prior to removal of theSDA from the SSZ-28) and in the anhydrous state, comprising thefollowing (in terms of mole ratios): (0.1 to 2) Q: (0 to 1.0) Z: (0.1 to2.0) M: X₂O₃: (20 to <50) YO₂ wherein Q is a is a structure directingagent selected from N,N-dimethyltropinium or N,N-dimethyl-3-azoniumbicyclo[3.2.2]nonane cations; Z is an amine component comprising atleast one amine chosen from amines having from 1 to 8 carbon atoms,ammonium hydroxide and mixtures thereof; M is an alkali metal cation; Xis selected from aluminum, gallium, iron, boron and mixtures thereof;and Y is selected from silicon, germanium and mixtures thereof. Asprepared, the YO₂/X₂O₃ mole ratio is typically in the range of 30 about45. In one embodiment, SSZ-28 is an aluminosilicate wherein X isaluminum and Y is silicon.

SSZ-28 can be characterized by its X-ray diffraction pattern. SSZ-28,as-synthesized, has a crystalline structure whose X-ray powderdiffraction pattern exhibits the characteristic lines shown in Table 1.

TABLE 1 As-Synthesized SSZ-28 2-Theta^((a)) d-Spacing RelativeIntegrated (degrees) (nm) Intensity (%)^((b)) 7.72 1.145 W 11.42 0.775 W15.02 0.679 M 15.40 0.615 W 15.46 0.573 VS 17.18 0.516 VS 18.33 0.484 S18.92 0.469 VS 19.73 0.450 S 26.28 0.339 VS 26.58 0.335 S 26.93 0.331 M^((a))±0.20 ^((b))The X-ray patterns provided are based on a relativeintensity scale in which the strongest line in the X-ray pattern isassigned a value of 100: W (weak) is less than 20; M (medium) is between20 and 40; S (strong) is between 40 and 60; VS (very strong) is greaterthan 60.

Crystalline SSZ-28 can be used as-synthesized, but preferably will bethermally treated (calcined). Usually, it is desirable to remove thealkali metal cation (if any) by ion exchange and replace it withhydrogen, ammonium, or any desired metal ion.

After calcination, the X-ray powder diffraction pattern for SSZ-28exhibits the characteristic lines shown in Table 2 below.

TABLE 2 Calcined SSZ-28 2-Theta^((a)) d-Spacing Relative Integrated(degrees) (nm) Intensity (%)^((b)) 6.56 1.350 VS^((c)) 7.79 1.135 VS11.45 0.773 VS 12.92 0.685 VS 14.47 0.612 VS 15.52 0.571 VS 17.25 0.514VS 18.41 0.482 VS 18.92 0.469 VS 19.86 0.447 S 26.30 0.338 VS 26.620.334 S 26.95 0.330 S ^((a))±0.20 ^((b))The X-ray patterns provided arebased on a relative intensity scale in which the strongest line in theX-ray pattern is assigned a value of 100: W (weak) is less than 20; M(medium) is between 20 and 40; S (strong) is between 40 and 60; VS (verystrong) is greater than 60. ^((c))Can have greatly varied intensity

The X-ray powder diffraction patterns were determined by standardtechniques. The radiation was CuKα radiation. The peak heights and thepositions, as a function of 2θ where θ is the Bragg angle were read fromthe relative intensities of the peaks, and d, the interplanar spacing innanometers corresponding to the recorded lines, can be calculated.

The variation in the scattering angle (two theta) measurements, due toinstrument error and to differences between individual samples, isestimated at ±0.20 degrees. Calcination can result in changes in theintensities of the peaks as compared to patterns of the “as-synthesized”material, as well as minor shifts in the diffraction pattern.

SSZ-28 can be formed into a wide variety of physical shapes. Generallyspeaking, the molecular sieve can be in the form of a powder, a granule,or a molded product, such as extrudate having a particle size sufficientto pass through a 2-mesh (Tyler) screen and be retained on a 400-mesh(Tyler) screen. In cases where the catalyst is molded, such as byextrusion with an organic binder, the SSZ-28 can be extruded beforedrying, or, dried or partially dried and then extruded.

SSZ-28 can be composited with other materials resistant to thetemperatures and other conditions employed in organic conversionprocesses. Such matrix materials include active and inactive materialsand synthetic or naturally occurring zeolites as well as inorganicmaterials such as clays, silica and metal oxides. Examples of suchmaterials and the manner in which they can be used are disclosed in U.S.Pat. No. 4,910,006 and U.S. Pat. No. 5,316,753.

SSZ-28 can be used for the catalytic reduction of the oxides of nitrogenin a gas stream. Typically, the gas stream also contains oxygen, often astoichiometric excess thereof. Also, the molecular sieve may contain ametal or metal ions within or on it which are capable of catalyzing thereduction of the nitrogen oxides. Examples of such metals or metal ionsinclude cobalt, copper, platinum, iron, chromium, manganese, nickel,zinc, lanthanum, palladium, rhodium and mixtures thereof.

One example of such a process for the catalytic reduction of oxides ofnitrogen in the presence of a zeolite is disclosed in U.S. Pat. No.4,297,328. There, the catalytic process is the combustion of carbonmonoxide and hydrocarbons and the catalytic reduction of the oxides ofnitrogen contained in a gas stream, such as the exhaust gas from aninternal combustion engine. The zeolite used is metal ion-exchanged,doped or loaded sufficiently so as to provide an effective amount ofcatalytic copper metal or copper ions within or on the zeolite. Inaddition, the process is conducted in an excess of oxidant, e.g.,oxygen.

EXAMPLES

The following examples are given to illustrate the present invention. Itshould be understood, however, that the invention is not to be limitedto the specific conditions or details described in these examples.

Example 1 Synthesis of SSZ-28

4.5 Grams of a 0.67 M solution of N,N-dimethyltropinium hydroxide asprepared according to Example 1 of U.S. Pat. No. 5,200,377 was mixedwith 6 mL of water and 0.103 g of KOH (solid). After dissolution, 2.36 gof LUDOX® AS-30 colloidal silica (30% SiO₂) was added with stirringusing a magnetic stir bar. Finally, 0.78 g of Nalco 1SJ612 alumina onsilica (30% solids, 4% Al₂O₃ overall) was added. The reactants wereloaded into a Parr 4745 reactor, sealed and loaded onto a rotating spitin a Blue M oven. The reactor was rotated at 30 rpm while being heatedat 175° C. for 6 days. The product (after filtration, washing withdistilled water, and drying in air and then at 100° C.) was thecrystalline material designated SSZ-28 as determined by powder X-raydiffraction.

Example 2 Calcination of SSZ-28

The material from Example 1 was heated in a muffle furnace from roomtemperature up to 540° C. at a steadily increasing rate over a 7 hourperiod. The sample was maintained at 540° C. for 4 more hours and thentaken up to 600° C. for an additional 4 hours. A 50/50 mixture of airand nitrogen was passed over the molecular sieve at a rate of 20standard cubic feet per minute during heating.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims, are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that can vary depending upon thedesired properties sought to be obtained by the present invention. It isnoted that, as used in this specification and the appended claims, thesingular forms “a,” “an,” and “the,” include plural references unlessexpressly and unequivocally limited to one referent. As used herein, theterm “include” and its grammatical variants are intended to benon-limiting, such that recitation of items in a list is not to theexclusion of other like items that can be substituted or added to thelisted items. As used herein, the term “comprising” means includingelements or steps that are identified following that term, but any suchelements or steps are not exhaustive, and an embodiment can includeother elements or steps.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope is defined bythe claims, and can include other examples that occur to those skilledin the art. Such other examples are intended to be within the scope ofthe claims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims. To an extent not inconsistent herewith, all citationsreferred to herein are hereby incorporated by reference.

The invention claimed is:
 1. A process for reduction of oxides ofnitrogen contained in a gas stream wherein the process comprisescontacting the gas stream with a crystalline molecular sieve having amole ratio of an oxide selected from silicon oxide, germanium oxide andmixtures thereof to an oxide selected from aluminum oxide, galliumoxide, iron oxide, boron oxide and mixtures thereof greater than about20:1 to about 45:1, and having, after calcination, the X-ray diffractionlines of Table 2, wherein the gas stream is an exhaust stream of aninternal combustion engine.
 2. The process of claim 1 conducted in thepresence of oxygen.
 3. The process of claim 1, wherein the molecularsieve contains a metal or metal ions capable of catalyzing the reductionof the oxides of nitrogen.
 4. The process of claim 3, wherein the metalis cobalt, copper, platinum, iron, chromium, manganese, nickel, zinc,lanthanum, palladium, rhodium or mixtures thereof.